[PATCH] improved TT scheduling for EHCI

This updates the EHCI driver by adding an improved scheduler for the
transaction translators, found in USB 2.0 hubs and used for low and
full speed devices.

 - adds periodic_tt_usecs() and some helper functions, which does
   the same thing that "periodic_usecs" does, except on the other
   side of the TT, i.e.  it calculates the low/fullspeed bandwidth
   usage instead of highspeed.

 - adds a tt_available() function which is the new implementation
   of what tt_no_collision() does ... while tt_no_collision() ensures
   that each TT handles only 1 periodic transfer at a time (a very
   pessimistic approach) this version instead tracks bandwidth and
   allows each TT to handle as many transfers as will fit on each TT's
   downstream bus (closer to best-case).

The new scheduler is selected by a config option, marked as EXPERIMENTAL
so it can be tested (and more broadly reviewed) for a while until it
seems safe to remove the original scheduler.

Signed-off-by: Dan Streetman <ddstreet@ieee.org>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/host/Kconfig

@@ -47,7 +47,25 @@ config USB_EHCI_ROOT_HUB_TT
 	  controller is needed.  It's safe to say "y" even if your
 	  controller doesn't support this feature.
 
-	  This supports the EHCI implementation from TransDimension Inc.
+	  This supports the EHCI implementation that's originally
+	  from ARC, and has since changed hands a few times.
+
+config USB_EHCI_TT_NEWSCHED
+	bool "Improved Transaction Translator scheduling (EXPERIMENTAL)"
+	depends on USB_EHCI_HCD && EXPERIMENTAL
+	---help---
+	  This changes the periodic scheduling code to fill more of the low
+	  and full speed bandwidth available from the Transaction Translator
+	  (TT) in USB 2.0 hubs.  Without this, only one transfer will be
+	  issued in each microframe, significantly reducing the number of
+	  periodic low/fullspeed transfers possible.
+
+	  If you have multiple periodic low/fullspeed devices connected to a
+	  highspeed USB hub which is connected to a highspeed USB Host
+	  Controller, and some of those devices will not work correctly
+	  (possibly due to "ENOSPC" or "-28" errors), say Y.
+
+	  If unsure, say N.
 
 config USB_ISP116X_HCD
 	tristate "ISP116X HCD support"

drivers/usb/host/ehci-sched.c

@@ -163,6 +163,190 @@ static int same_tt (struct usb_device *dev1, struct usb_device *dev2)
 		return 1;
 }
 
+#ifdef CONFIG_USB_EHCI_TT_NEWSCHED
+
+/* Which uframe does the low/fullspeed transfer start in?
+ *
+ * The parameter is the mask of ssplits in "H-frame" terms
+ * and this returns the transfer start uframe in "B-frame" terms,
+ * which allows both to match, e.g. a ssplit in "H-frame" uframe 0
+ * will cause a transfer in "B-frame" uframe 0.  "B-frames" lag
+ * "H-frames" by 1 uframe.  See the EHCI spec sec 4.5 and figure 4.7.
+ */
+static inline unsigned char tt_start_uframe(struct ehci_hcd *ehci, __le32 mask)
+{
+	unsigned char smask = QH_SMASK & le32_to_cpu(mask);
+	if (!smask) {
+		ehci_err(ehci, "invalid empty smask!\n");
+		/* uframe 7 can't have bw so this will indicate failure */
+		return 7;
+	}
+	return ffs(smask) - 1;
+}
+
+static const unsigned char
+max_tt_usecs[] = { 125, 125, 125, 125, 125, 125, 30, 0 };
+
+/* carryover low/fullspeed bandwidth that crosses uframe boundries */
+static inline void carryover_tt_bandwidth(unsigned short tt_usecs[8])
+{
+	int i;
+	for (i=0; i<7; i++) {
+		if (max_tt_usecs[i] < tt_usecs[i]) {
+			tt_usecs[i+1] += tt_usecs[i] - max_tt_usecs[i];
+			tt_usecs[i] = max_tt_usecs[i];
+		}
+	}
+}
+
+/* How many of the tt's periodic downstream 1000 usecs are allocated?
+ *
+ * While this measures the bandwidth in terms of usecs/uframe,
+ * the low/fullspeed bus has no notion of uframes, so any particular
+ * low/fullspeed transfer can "carry over" from one uframe to the next,
+ * since the TT just performs downstream transfers in sequence.
+ *
+ * For example two seperate 100 usec transfers can start in the same uframe,
+ * and the second one would "carry over" 75 usecs into the next uframe.
+ */
+static void
+periodic_tt_usecs (
+	struct ehci_hcd *ehci,
+	struct usb_device *dev,
+	unsigned frame,
+	unsigned short tt_usecs[8]
+)
+{
+	__le32			*hw_p = &ehci->periodic [frame];
+	union ehci_shadow	*q = &ehci->pshadow [frame];
+	unsigned char		uf;
+
+	memset(tt_usecs, 0, 16);
+
+	while (q->ptr) {
+		switch (Q_NEXT_TYPE(*hw_p)) {
+		case Q_TYPE_ITD:
+			hw_p = &q->itd->hw_next;
+			q = &q->itd->itd_next;
+			continue;
+		case Q_TYPE_QH:
+			if (same_tt(dev, q->qh->dev)) {
+				uf = tt_start_uframe(ehci, q->qh->hw_info2);
+				tt_usecs[uf] += q->qh->tt_usecs;
+			}
+			hw_p = &q->qh->hw_next;
+			q = &q->qh->qh_next;
+			continue;
+		case Q_TYPE_SITD:
+			if (same_tt(dev, q->sitd->urb->dev)) {
+				uf = tt_start_uframe(ehci, q->sitd->hw_uframe);
+				tt_usecs[uf] += q->sitd->stream->tt_usecs;
+			}
+			hw_p = &q->sitd->hw_next;
+			q = &q->sitd->sitd_next;
+			continue;
+		// case Q_TYPE_FSTN:
+		default:
+			ehci_dbg(ehci,
+				  "ignoring periodic frame %d FSTN\n", frame);
+			hw_p = &q->fstn->hw_next;
+			q = &q->fstn->fstn_next;
+		}
+	}
+
+	carryover_tt_bandwidth(tt_usecs);
+
+	if (max_tt_usecs[7] < tt_usecs[7])
+		ehci_err(ehci, "frame %d tt sched overrun: %d usecs\n",
+			frame, tt_usecs[7] - max_tt_usecs[7]);
+}
+
+/*
+ * Return true if the device's tt's downstream bus is available for a
+ * periodic transfer of the specified length (usecs), starting at the
+ * specified frame/uframe.  Note that (as summarized in section 11.19
+ * of the usb 2.0 spec) TTs can buffer multiple transactions for each
+ * uframe.
+ *
+ * The uframe parameter is when the fullspeed/lowspeed transfer
+ * should be executed in "B-frame" terms, which is the same as the
+ * highspeed ssplit's uframe (which is in "H-frame" terms).  For example
+ * a ssplit in "H-frame" 0 causes a transfer in "B-frame" 0.
+ * See the EHCI spec sec 4.5 and fig 4.7.
+ *
+ * This checks if the full/lowspeed bus, at the specified starting uframe,
+ * has the specified bandwidth available, according to rules listed
+ * in USB 2.0 spec section 11.18.1 fig 11-60.
+ *
+ * This does not check if the transfer would exceed the max ssplit
+ * limit of 16, specified in USB 2.0 spec section 11.18.4 requirement #4,
+ * since proper scheduling limits ssplits to less than 16 per uframe.
+ */
+static int tt_available (
+	struct ehci_hcd		*ehci,
+	unsigned		period,
+	struct usb_device	*dev,
+	unsigned		frame,
+	unsigned		uframe,
+	u16			usecs
+)
+{
+	if ((period == 0) || (uframe >= 7))	/* error */
+		return 0;
+
+	for (; frame < ehci->periodic_size; frame += period) {
+		unsigned short tt_usecs[8];
+
+		periodic_tt_usecs (ehci, dev, frame, tt_usecs);
+
+		ehci_vdbg(ehci, "tt frame %d check %d usecs start uframe %d in"
+			" schedule %d/%d/%d/%d/%d/%d/%d/%d\n",
+			frame, usecs, uframe,
+			tt_usecs[0], tt_usecs[1], tt_usecs[2], tt_usecs[3],
+			tt_usecs[4], tt_usecs[5], tt_usecs[6], tt_usecs[7]);
+
+		if (max_tt_usecs[uframe] <= tt_usecs[uframe]) {
+			ehci_vdbg(ehci, "frame %d uframe %d fully scheduled\n",
+				frame, uframe);
+			return 0;
+		}
+
+		/* special case for isoc transfers larger than 125us:
+		 * the first and each subsequent fully used uframe
+		 * must be empty, so as to not illegally delay
+		 * already scheduled transactions
+		 */
+		if (125 < usecs) {
+			int ufs = (usecs / 125) - 1;
+			int i;
+			for (i = uframe; i < (uframe + ufs) && i < 8; i++)
+				if (0 < tt_usecs[i]) {
+					ehci_vdbg(ehci,
+						"multi-uframe xfer can't fit "
+						"in frame %d uframe %d\n",
+						frame, i);
+					return 0;
+				}
+		}
+
+		tt_usecs[uframe] += usecs;
+
+		carryover_tt_bandwidth(tt_usecs);
+
+		/* fail if the carryover pushed bw past the last uframe's limit */
+		if (max_tt_usecs[7] < tt_usecs[7]) {
+			ehci_vdbg(ehci,
+				"tt unavailable usecs %d frame %d uframe %d\n",
+				usecs, frame, uframe);
+			return 0;
+		}
+	}
+
+	return 1;
+}
+
+#else
+
 /* return true iff the device's transaction translator is available
  * for a periodic transfer starting at the specified frame, using
  * all the uframes in the mask.
@@ -237,6 +421,8 @@ static int tt_no_collision (
 	return 1;
 }
 
+#endif /* CONFIG_USB_EHCI_TT_NEWSCHED */
+
 /*-------------------------------------------------------------------------*/
 
 static int enable_periodic (struct ehci_hcd *ehci)
@@ -481,7 +667,7 @@ static int check_intr_schedule (
 )
 {
     	int		retval = -ENOSPC;
-	u8		mask;
+	u8		mask = 0;
 
 	if (qh->c_usecs && uframe >= 6)		/* FSTN territory? */
 		goto done;
@@ -494,6 +680,24 @@ static int check_intr_schedule (
 		goto done;
 	}
 
+#ifdef CONFIG_USB_EHCI_TT_NEWSCHED
+	if (tt_available (ehci, qh->period, qh->dev, frame, uframe,
+				qh->tt_usecs)) {
+		unsigned i;
+
+		/* TODO : this may need FSTN for SSPLIT in uframe 5. */
+		for (i=uframe+1; i<8 && i<uframe+4; i++)
+			if (!check_period (ehci, frame, i,
+						qh->period, qh->c_usecs))
+				goto done;
+			else
+				mask |= 1 << i;
+
+		retval = 0;
+
+		*c_maskp = cpu_to_le32 (mask << 8);
+	}
+#else
 	/* Make sure this tt's buffer is also available for CSPLITs.
 	 * We pessimize a bit; probably the typical full speed case
 	 * doesn't need the second CSPLIT.
@@ -514,6 +718,7 @@ static int check_intr_schedule (
 			goto done;
 		retval = 0;
 	}
+#endif
 done:
 	return retval;
 }
@@ -1047,12 +1252,21 @@ sitd_slot_ok (
 		frame = uframe >> 3;
 		uf = uframe & 7;
 
+#ifdef CONFIG_USB_EHCI_TT_NEWSCHED
+		/* The tt's fullspeed bus bandwidth must be available.
+		 * tt_available scheduling guarantees 10+% for control/bulk.
+		 */
+		if (!tt_available (ehci, period_uframes << 3,
+				stream->udev, frame, uf, stream->tt_usecs))
+			return 0;
+#else
 		/* tt must be idle for start(s), any gap, and csplit.
 		 * assume scheduling slop leaves 10+% for control/bulk.
 		 */
 		if (!tt_no_collision (ehci, period_uframes << 3,
 				stream->udev, frame, mask))
 			return 0;
+#endif
 
 		/* check starts (OUT uses more than one) */
 		max_used = 100 - stream->usecs;